The present disclosure relates generally to the field of high resolution medical imaging. More particularly, the present disclosure relates to minimally invasive methods involving two or more imaging modalities.
High resolution medical imaging has broad diagnostic utility, including assessing tissue structures, anatomy and/or composition, planning and/or guiding interventions on localized regions of the body, and assessing the result of interventions that alter the structure, composition or other properties of the localized region. Among the many different high resolution imaging modalities, high frequency ultrasound and optical coherence tomography are two highly useful clinical and research tools.
High frequency ultrasound is a technique that is particularly useful for intravascular and intracardiac procedures. For these applications, one or more ultrasound transducers are incorporated into a catheter or other device that can be inserted into the body. Two particularly important implementations of high frequency ultrasound are intravascular ultrasound (IVUS), for imaging blood vessels, and intracardiac echocardiography (ICE) for imaging cardiac chambers. Both ICE and IVUS are minimally invasive, and involve placing one or more ultrasound transducers inside a blood vessel or cardiac chamber to take high quality images of these structures.
The center frequency of IVUS typically lies within the range of 3 to 200 MHz and more typically in the range of 8 to 80 MHz. Higher frequencies provide higher resolution but result in worse signal penetration and thus a smaller field of view. Depth of penetration can range from less than a millimeter to several centimeters depending on several parameters such as center frequency and geometry of the transducer, the attenuation of the media through which the imaging occurs and implementation-specific specifications that affect the signal to noise ratio of the system.
High resolution imaging methods often involve the use of a rotary shaft to transmit torque to an imaging device near the distal end of the probe. These rotary shafts are often long, thin and flexible so that they can be delivered through anatomical conduits, such as the vasculature, genitourinary tracts, respiratory tracts and other such bodily lumens. Ideally, when torque is applied to the cable in a specified direction the torque cable develops a property of having a close relation between the degree of rotation at its proximal and distal ends. This allows the simplification of the design of an ultrasound catheter by making the angle of rotation at the distal end of the torque cable (within the body) a reasonable approximation of the angle of rotation at the proximal end of the torque cable (outside of the body).
Other imaging systems operate without a torque cable, such as angioscopy catheters (which employ fiber optic bundles) and phased array imaging systems. Additionally, imaging systems have been proposed and demonstrated that incorporate a micro-motor in the distal end of the catheter instead of relying on a torque cable.
Variations of high frequency ultrasound exist, where the signal acquisition and/or analysis of the backscattered signal is modified to facilitate obtaining or inferring further information about the imaged tissue. These include elastography, where the strain within tissue is assessed as the tissue is compressed at different blood pressures (de Korte et al Circulation. 2002 Apr. 9; 105(14):1627-30); Doppler imaging which assesses motion such as blood flow within anatomic structures; virtual histology, which attempts to infer the composition of tissue using the radio-frequency properties of the backscattered signal combined with a pattern recognition algorithm (Nair, U.S. Pat. No. 6,200,268); second harmonic imaging (Goertz et al, Invest Radiol. 2006 August; 41(8):631-8) and others. Ultrasound transducers are improving considerably, including the use of single crystal ultrasound transducers and composite ultrasound transducers.
A catheter-based system for intravascular ultrasound is described by Yock (U.S. Pat. No. 4,794,931) to provide high resolution imaging of structures in blood vessels. This system comprises an outer sheath, within which there is an ultrasound transducer near the distal end of a long torque cable. When a motor rotates the torque cable and ultrasound transducer assembly, 2D cross-sectional images of anatomical structures, such as blood vessels, can be made. Linear translation of the catheter or the torque cable and ultrasound transducer in combination with the rotational motion of the ultrasound transducer allows for acquisition of a series of 2D images along the length of the catheter.
Hossack et al (WO/2006/121851) describe a forward looking ultrasound transducer using a CMUT transducer and a reflective surface.
Optical imaging methods based on fiber optic technology used in the field of medicine include optical coherence tomography (OCT), angioscopy, near infrared spectroscopy, Raman spectroscopy and fluorescence spectroscopy. These modalities typically require the use of one or more optical fibers to transmit light energy along a shaft between an imaging site and an imaging detector.
Optical coherence tomography is an optical analog of ultrasound, and provides imaging resolutions on the order of 1 to 30 microns, but does not penetrate as deeply into tissue as ultrasound in most cases. Fiber optics can also be used to deliver energy for therapeutic maneuvers such as laser ablation of tissue and photodynamic therapy. Other useful optical imaging modalities include endoscopy and other similar or related imaging mechanisms that involve the use of a probe to obtain images based on the back-reflection of light. Miniaturization of detectors and light sources is making it possible to include the light sources and/or detectors in the catheter itself, potentially obviating the need for fiber optics to act as an intermediary component in the transmission and/or detection of light.
Optical coherence tomography is limited by its small penetration depth (on the order of 500 to 3000 microns) in most biologic media. Most such media, including blood, are not optically transparent. OCT has thus far required the displacement of blood to create an optically clear environment for this purpose. One approach is to displace the blood with another fluid prior to performing measurements with the imaging modality incompatible with blood. U.S. Pat. No. 7,625,366, issued to Atlas, provides an exemplary flush catheter for injecting a flush solution into a vessel for performing OCT measurements with minimal blood displacement. Fluids that have been either used or contemplated for this purpose include radio-opaque contrast or various formulations of saline, Ringer's lactate and others. U.S. Pat. No. 7,794,446 (issued to Bosse et al.) and U.S. Pat. No. 7,747,315 (issued to Villard et al.) disclose improved flush solution compositions for use in OCT imaging.
Displacement of blood by the introduction of another fluid with greater transparency provides a time interval in which optical coherence tomography imaging can occur. This time window can be extended by reducing the flow within the vessel, such as by the use guide catheters that incorporate an occlusion balloon. For example, U.S. Pat. Nos. 5,722,403, 5,740,808, 5,752,158, 5,848,969, 5,904,651, and 6,047,218, issued to McGee et al., provide imaging catheter systems including an inflatable balloon that incorporates an imaging apparatus. U.S. Pat. No. 7,674,240, issued to Webler et al., provides improved devices for inflating and deflating balloons for occluding a vessel.
Displacement of blood by means of introduction of another fluid to improve OCT imaging is conventionally done by a manual process, where the operator injects the transparent fluid on one or more occasions during an imaging procedure. Such injection may be done via a number of methods, including use of a manual syringe, use of pressurized fluid delivery systems and use of powered pumps. Pressurized fluid delivery systems can include the simple use of gravitational forces to provide pressure, as well as devices that apply pressure to a compressible or deformable compartment filled with the fluid of interest. For example, pressure infuser bags use an inflatable bladder, similar to that of a conventional blood pressure cuff, to apply pressure to a bag of fluid within a confined compartment. The inflatable bladder and the bag of fluid share a confined space. Therefore, when the bladder is inflated, such as with a manual hand pump, pressurized infusions of fluid into a patient is possible.
Alternatively, blood can be displaced by use of a balloon filled with an optically clear medium, such as radio-opaque contrast, saline or air. The balloon may surround the region of the catheter where light, such as that used for OCT imaging or near infra-red (NIR) spectroscopy, exits the imaging probe.
Unfortunately, complications can arise when displacing blood from a vessel. For example, there is a small risk of embolic events, if the introduction of displaced fluid dislodges particles from the vessel wall. There is a risk of causing or worsening a dissection between the layers of the vessel wall if fluid is injected inadvertently with too much force, or if the fluid is injected near a pre-existing dissection site. In critical organs such as the heart, the potential complications of displacing blood with another fluid include ischemia to the target organ and arrhythmias. Cardiac arrhythmias may occur as a result of hypoxia if the displacing fluid does not carry adequate oxygen to the myocardium. They may also occur due to changes in the concentrations of electrolytes in the myocardium.
For vessels that perfuse critical organs sensitive to hypoxia, such as the heart, brain and kidneys, prolonged intervals of blood displacement and/or vascular occlusion can lead to adverse clinical events, and the operator may be compelled to minimize the duration of time over which the displacement of blood occurs.
The need to minimize the amount of time during which blood is displaced has to be balanced with the desire to acquire an adequate amount of imaging data. For example, if the imaging probe is translated along the vessel's longitudinal axis, the portion of the vessel adequately imaged by an optical imaging technique will be limited by the length of time during which blood is displaced adequately. Not only is the time duration over which blood is displaced of importance, but if an injection of an optically transmissive fluid is being used, then the volume of fluid injected may have important consequences. For example, some operators use radio-opaque contrast as the optically transmissive medium. Yet it is well known in the field of medicine that contrast agents frequently have deleterious effects on kidney function and can contribute to acute renal failure. Conversely, inadequate displacement of blood results in sub-optimal imaging.
Variations of optical coherence tomography (OCT) include polarization sensitive OCT (PS-OCT) where the birefringent properties of tissue components can be exploited to obtain additional information about structure and composition; spectroscopic OCT which similarly provides improved information regarding the composition of the imaged structures; Doppler OCT which provides information regarding flow and motion; elastography via OCT; and optical frequency domain imaging (OFDI), which allows for a markedly more rapid acquisition of imaging data and therefore enables imaging to occur over a larger volume of interest in less time.
There exist several other forms of fiber-optic based imaging other than OCT. Amundson et al describe a system for imaging through blood using infrared light (U.S. Pat. No. 6,178,346). The range of the electromagnetic spectrum that is used for their imaging system is selected to be one which optimizes penetration through blood, allowing optical imaging through blood similar to that afforded by angioscopy in the visible spectrum, but without the need to flush blood away from the region being imaged.
Tearney et al (U.S. Pat. No. 6,134,003) describe several embodiments that enable optical coherence tomography to provide higher resolution imaging than is readily obtained by high frequency ultrasound or IVUS.
Dewhurst (U.S. Pat. No. 5,718,231) discloses a forward looking probe for intravascular imaging where a fiber optic travels through an ultrasound transducer to shine light on a target tissue straight in front of the end of the probe. The light then interacts with the target tissue and makes ultrasound waves, which are received by the ultrasound sensor and the images are photoacoustic images only as the system is not configured to receive and process optical images. The ultrasound sensor used in the Dewhurst device is limited to thin film polymeric piezoelectrics, such as thin film PVDF, and is used only to receive ultrasound energy, not to convert electrical energy to ultrasound.
Angioscopy, endoscopy, bronchoscopy and many other imaging devices have been described which allow for the visualization of internal conduits and structures (such as vessels, gastrointestinal lumens and the pulmonary system) in mammalian bodies based on the principle of illuminating a region within the body near the distal end of a rigid or flexible shaft. Images are then created by either having a photodetector array (such as a CCD array) near the end of the shaft or by having a bundle of fiber optics transmit the received light from the distal end of the shaft to the proximal end where a photodetector array or other system that allows the operator to generate or look at an image representative of the illuminated region. Fiber bundles are bulky and reduce the flexibility of the shaft among other disadvantages.
Other fiber optic based modalities for minimally invasive assessment of anatomic structures include Raman spectroscopy as described by Motz et al. (J Biomed Opt. 2006 March-April; 11(2)), near infrared spectroscopy as described by Caplan et al (J Am Coll Cardiol. 2006 Apr. 18; 47(8 Suppl):C92-6) and fluorescence imaging, such as tagged fluorescent imaging of proteolytic enzymes in tumors (Radiology. 2004 June; 231(3):659-66).
Recently, probe designs have emerged that combine multiple imaging modalities in a single device. Maschke (United States Patent Publication No. 2006/0116571 corresponding to U.S. patent application Ser. No. 11/291,593) describes an embodiment of a guidewire with both OCT and IVUS imaging transducers mounted upon it. The described invention has several shortcomings. Guidewires are typically 0.014″ to 0.035″ in diameter (approximately 350 microns to 875 microns), yet ultrasound transducers typically are at least 400 microns×400 microns and generally are larger in size for the frequencies in the 20 to 100 MHz range. If the transducer is too small, the beam is poorly focused and has poor signal properties. In Maschke, the IVUS and OCT imaging mechanisms are located at different positions along the length of the guidewire, and a substantial drawback associated with this type of configuration (having the IVUS and OCT imaging means located at different positions along the length of an imaging shaft) is that optimal co-registration of images is not possible.
Similarly, U.S. Pat. No. 7,289,842 issued to Maschke describes an imaging system that combines IVUS and OCT on a catheter where the IVUS and OCT imaging elements are longitudinally displaced from each other along the length of a catheter that rotates around its longitudinal axis. Maschke also describes generating images where the center portion of the images are substantially derived from the output of the higher resolution OCT imaging portion of the system while the outer portion of the images are substantially derived from the output of the ultrasound imaging portion of the system, to make use of ultrasound's greater depth of penetration in combination with OCT's higher resolution for tissues close to the catheter.
U.S. Pat. No. 6,390,978, issued to Irion, describes the use of high frequency ultrasound in combination with optical coherence tomography where the ultrasound beam and the OCT beam are superimposed on each other.
In U.S. Patent Application Publication No. 2008/0177138, Courtney et al. provide an improved multimodal imaging system incorporating both IVUS and OCT transducers in a compact imaging assembly capable of side-viewing and/or forward-looking imaging. Such multimodal imaging systems offer the ability to obtain far greater diagnostic information than using a single modality imaging device. Indeed, optical coherence tomography generally has superior resolution to ultrasound and has the potential to better identify some structures or components in vascular and other tissues than ultrasound. For example, fibrous cap thickness or the presence of inflammatory or necrotic regions near the surface of arteries may be better resolved with optical coherence tomography.
Unfortunately, many multimodal imaging devices suffer from problems related to incompatibility of one or more imaging modalities with blood. For example, in the case of a multimodal imaging device combining both IVUS and OCT, the IVUS transducer is capable of functioning with the presence of blood in the vessel under investigation, but the OCT modality requires blood displacement. Such a requirement leads to complexity of operation and difficulties in coordinating and referencing the results from the two imaging modalities.
Another problem with the use of multimodal imaging devices is the inaccuracies in co-registration that might result when one imaging modality is used, followed by another imaging modality after blood displacement. For example, intravascular imaging, such as IVUS and OCT, is often used for clinical trial purposes where an imaging protocol is required. Manually using one or more modalities to identify regions that should be assessed in greater detail by one or more other modality is subject to a substantial amount of operator variability. Furthermore, clinical studies that depend on the ability to compare the structure and/or composition of vessels between different patients or at different time points will be dependent on reproducible methods for assessment.
In U.S. Pat. No. 7,758,499, Adler teaches the use of IR imaging with wavelengths of less than 1000 nm, which is minimally compromised by the presence of blood, in combination with other imaging modalities, such as imaging with visible light. To achieve multimodal optical imaging, blood displacement methods are employed, enabling imaging with IR and/or visible light.
The use of multiple imaging modalities in a single imaging device was also recently described by Muller et al. (US Patent Application Publication No. 2009/0299195). Muller describes methods and systems for combining intravascular ultrasound, optical coherence tomography, and near infrared spectroscopy for the detection of multiple, different abnormalities in the arterial morphology during a single intravascular procedure.
Unfortunately, the known methods employing manual operations for serially acquiring multimodal images require considerable skill and further involve complex image spatial alignment operations. Accordingly, there remains a need for multimodal imaging methods that address the aforementioned problems, enable standardized image data acquisition, and provide improved performance and clinical utility.